The United Kingdom has no fundamental constitutional instrument. It is in that respect almost unique. Instead it has a fundamental constitutional doctrine: the doctrine of the sovereignty of Parliament. The first paradox of the United Kingdom constitution is that no rules have a constitutional status.
The doctrine of Parliamentary sovereignty entails that all the constitutional rules that, in other countries, would be set out in a constitution are, in the United Kingdom, contained in Acts of Parliament-or in the common law, or in unwritten constitutional conventions or custom; and that any such rules, whether statutory or not, can be repealed or amended by an ordinary Act of Parliament, with no special procedure and no special majority being required. No provisions have constitutional status; still less are any provisions entrenched. There is no "fundamental law."
Francis G. Jacobs,
Towards a United Kingdom Bill of Rights,
U. Mich. J. L. Reform
Available at: https://repository.law.umich.edu/mjlr/vol18/iss1/3